1. Field of the Invention
The present invention relates to an organic electrolulminescence (EL) element, and more particularly to prevention of attenuation of light to be externally emitted.
2. Description of Related Art
Organic EL display panels (organic EL display apparatuses) have conventionally been known as one example of flat display panels. Because, unlike liquid crystal display (LCD) panels, organic EL display apparatuses are self emissive and because organic EL display apparatuses are bright and clear flat display panels, their widespread use is very much expected.
An organic EL display apparatus comprises a large number of organic EL elements arranged in a matrix, and employs these organic EL elements as pixels of a display. Such organic EL elements can be driven passively or actively, similar to LCDs, and, as is also the case with LCDS, active matrix displays are more preferable. More specifically, in active matrix displays, switching elements (typically two elements including a switching element and a driving element) are provided for each pixel and display of each pixel is controlled by controlling the switching elements, whereas in passive driving, a switching element is not provided for each pixel. Of these two types of displays, the active matrix is more preferable because much more precise displays can be achieved.
In such active matrix organic EL display apparatuses, it is necessary to provide, for each pixel, two switching elements, a capacitor, and an EL element, which are located at different positions on a plane. Typically, thin film transistors (TFTs) serving as switching elements and a capacitor are formed on a glass substrate, and an organic EL element formed by lamination of an anode such as ITO, a hole transport layer, an organic emissive layer, an electron transport layer, and a cathode is provided above the TETs and the capacitor.
When the TFT is driven to cause current to flow in the organic EL element, light produced in the emissive layer is emitted through the anode and the glass substrate. This is called bottom emission type, because light is emitted from the glass substrate.
As described above, the organic EL element is formed, after formation of the TFTs, above the TFTs. In this case, for the purpose of insulation between the TFTs and the organic EL element and because it is desired that the surface on which the organic EL element is formed be as flat as possible, a gate insulating film located between the gate electrode and the channel electrode of the TFT and an inter-layer insulating film covering the gate electrode are formed so as to cover the entire surface of the substrate. Then, a planarization film is formed to cover these layers and planarize the entire surface, and the anode is provided thereon.
Consequently, the planarization film, the inter-layer insulating film, the gate insulating film, or the like are provided between the anode of the organic EL element and the substrate located below. The planarization film is formed of an organic material such as an acrylic resin, and the inter-layer insulating film and the gate insulating film are formed of silicon oxide, silicon nitride, and so on. Thus, a laminated structure made up of layers of various materials is provided under the organic EL element.
Here, light emission is not reduced when the refractive indexes of these layers of various materials are substantially the same. However, when there is a significant difference in the refractive indexes between adjacent layers among these layers, significant reflection is caused at the interface of these layers. Consequently, in a structure in which light from the organic EL element is externally emitted through the substrate, there is a problem that the amount of light from the EL element to be transmitted decreases, and this results in reduction in the emission efficiency (external emission efficiency), which defines actually emitted light.